(1) Field of the Invention
The present invention relates to a near infrared absorption filter. More particularly, the present invention relates to a near infrared absorption filter which is suitably used particularly as a near infrared filter for a plasma display panel by combining with a layer having a different function.
(2) Description of the Prior Art
In recent years, large displays of various types have been developed and commercialized. A plasma display is one of such displays. A plasma display generates a near infrared light during the plasma discharge, as easily understood from the operational mechanism; and since remote control systems of household electronic appliances (such as a TV, air-conditioner, video tape recorder, etc.) use the same or similar wavelength of the near infrared region, a plasma display may cause false functioning of those appliances placed nearby.
Hence, it was proposed to utilize a near infrared absorption filter which can absorb and shield a light of the near infrared region, i.e. 800 to 1,000 nm, particularly 850 to 1,000 nm. As such a near infrared absorption filter, there can be mentioned, for example, (1) a filter made of a phosphate glass containing a metal ion (e.g. bivalent copper ion), (2) a filter obtained by forming, on a substrate (e.g. a glass), a thin layer of metal (e.g. silver) by vapor deposition, sputtering, ion-plating or any other methods, (3) a filter made of a phosphoric acid group-containing acrylic resin containing a metal ion (e.g. bivalent copper ion), and (4) a filter obtained by adding, to a resin, a dye capable of absorbing a light of the near infrared region.
The above near infrared absorption filters, however, have respective problems. The filter (1) made of a phosphate glass containing a metal ion is hygroscopic and employs a complicated production process. The filter (2) obtained by forming, on a substrate (e.g. a glass), a thin layer of metal (e.g. silver) by vapor deposition, ion-plating or any other methods, has the problem of reflecting not only the light of the near infrared region but of the visible region although its amount is smaller than that in near infrared region. And also it lowers its transmittance when it is too thick, and has a high production cost as well. In the filters (1) and (2), a glass is used in most cases; therefore, the filters are heavy, crack easily, and are difficult to mold.
For the filter (3) made of a phosphoric acid group-containing acrylic resin containing a metal ion, complexity of the production process, etc. is pointed out.
In contrast, the filter (4) obtained by adding, to a resin, a dye capable of absorbing a light of the near infrared region has various advantages such as light weight as compared with glass-made filters and easy production. However, the filter (4) has the following problems.
That is, many of the dyes capable of absorbing a light of the near infrared region show absorption also in a visible light region. Therefore, the filter (4) containing such a dye may possibly show absorption also in a visible light region other than the near infrared region, which is not preferred.
Further, many of the dyes capable of absorbing in the near infrared region, particularly many of the diimmonium compounds are inferior in heat resistance and heat stability; therefore, a near infrared absorption filter containing such a dye, produced, for example, by incorporation of the dye, often contains a heat decomposition product of incorporated dye. This heat decomposition product, unlike the incorporated dye per se, absorbs substantially no near infrared region but absorbs a visible light region. As a result, in the filter, the absorption of the near infrared region is relatively low as compared with the absorption of the visible light region, and the transmission in the visible light region becomes low.
Furthermore, the dyes capable of absorbing in the near infrared region, particularly the diimmonium compounds deteriorate faster when used in combination with other dyes. Therefore, in determining the dyes to be used in a near infrared absorption filter, it is necessary that they are selected so as to have no interaction with each other or they are used in a plurality of layers each containing a different dye.